Generally, the conventional type of refrigeration apparatus performs a vapor compression refrigeration cycle by circulating refrigerant in a refrigerant circuit. As such a type of refrigeration apparatus, one that performs a two-stage compression refrigeration cycle (i.e., the stage of compression of refrigerant is divided into two phases) has been known in the past.
The refrigeration apparatus of the two-stage compression refrigeration cycle type is provided with a lower stage compression mechanism and a higher stage compression mechanism. Gas refrigerant of low pressure from an evaporator is drawn into a compressor of the lower stage compression mechanism where the low-pressure gas refrigerant is compressed to an intermediate level of pressure. The refrigerant discharged from the lower stage compression mechanism is delivered to a compressor of the higher stage compression mechanism where the refrigerant is compressed to a further extent. And, the refrigerant discharged from the higher stage compression mechanism is delivered to a condenser.
However, such a type of refrigeration apparatus requires some arrangement (for example, such as the provision of an oil separator or an oil return passageway) in order that there may be no lack of refrigeration oil in the compressor of the higher stage compression mechanism as well as in the compressor of the lower stage compression mechanism. For example, Patent Document 1 discloses that an oil separator is provided for the separation of refrigeration oil from refrigerant discharged from the higher stage compression mechanism, and in addition, an oil return passageway is provided through which to return the separated refrigeration oil back to each of the higher and the lower stage compression mechanisms from the oil separator. In addition, Patent Document 2 discloses that a gas-liquid separator is provided for the separation of refrigeration oil from refrigerant discharged from the lower stage compression mechanism, and in addition, an oil return passageway is provided through which to return the separated refrigeration oil back to the lower stage compression mechanism from the gas-liquid separator.
Patent Document 1: JP-A-H07-260263
Patent Document 2: WO 02/46663